A solid polymer type fuel cell, in general, is comprised of a proton conducting electrolytic membrane sandwiched between an anode formed by a catalyst layer and a cathode formed by a catalyst layer. Furthermore, these are sandwiched between gas diffusion layers at their outsides. Furthermore, separators are arranged at the outsides of the same. This structure constitutes the basic structure. Such a basic structure is called a “unit cell”. Further, a fuel cell is usually comprised of a number of unit cells required for achieving the required output stacked together.
To take out current from such a basic structure of a solid polymer type fuel cell (unit cell), gas flow paths of the separators arranged at the anode side and cathode side supply the cathode side with oxygen or air or other oxidizing gas and, further, the anode side with hydrogen or other reducing gas. The thus supplied oxidizing gas and reducing gas are supplied through these gas diffusion layers to the catalyst layers. The energy difference (potential difference) between the chemical reaction occurring at the catalyst layer of the anode and the chemical reaction occurring at the catalyst layer of the cathode is utilized to take out current. For example, when using hydrogen gas and oxygen gas, the energy difference (potential difference) between the chemical reaction occurring on the metal catalyst particles of the catalyst layer of the anode [H2→2H++2e−(E0=0V)] and the chemical reaction occurring on the metal catalyst particles of the catalyst layer of the cathode [O2+4H++4e−→2H2O(E0=1.23V)] is taken out as current.
Therefore, unless the gas diffusion paths through which the oxidizing gas or reducing gas moves from the gas flow paths of a separator to the metal catalyst particles inside the catalyst layer at the cathode side or anode side, the proton conducting paths through which protons (H+) generated on the metal catalyst particles of the anode catalyst layer pass through the proton conducting electrolytic membrane and move to the metal catalyst particles of the cathode catalyst layer, and, furthermore, the electron conducting paths through which the electrons (e−) generated on the metal catalyst particles of the anode catalyst layer pass through the gas diffusion layer, separator, and outside circuit to move to the metal catalyst particles of the cathode catalyst layer continue to be connected without being split, current cannot efficiently be taken out.
Further, inside a catalyst layer, in general, it is also important that the pores formed in the clearances of the component materials and forming diffusion paths for the oxidizing gas or reducing gas, the electrolyte materials forming the proton conducting paths, and the carbon material or metal material or other conductive material forming the electron conducting paths be connected to form a network.
Further, for the proton conducting electrolytic membrane or proton conducting path in the catalyst layer (proton conducting electrolyte of ionomer), a polymer electrolyte material comprised of an ion exchange resin such as a perfluorosulfonic acid polymer is being used, but, in general, such a polymer electrolyte material first exhibits high proton conductivity in a moist environment. In a dry environment, the proton conductivity ends up falling. Therefore, to make a fuel cell operate efficiently, the polymer electrolyte material has to be maintained in a sufficiently moist state. Water vapor is supplied together with the gas supplied to the cathode side or anode side to maintain it in a moistened condition.
However, under such a moistened condition, the H2O generated on the metal catalyst particles of the cathode catalyst layer obstructs the diffusion of oxidizing gas at the gas diffusion paths in the cathode catalyst layer. As a result, the phenomenon arises of the cell performance of the solid polymer type fuel cell falling (flooding). For example, NPLT 1 describes that the hydrophilic functional groups of the carbon carrier adsorb H2O and that the H2O easily remains in the catalyst layer thereby causing a drop in the diffusion of the oxygen gas and is believed to be a factor behind flooding.
Further, in general, to obtain a solid polymer type fuel cell excellent in cell performance, it is necessary that the metal catalyst particles in the metal catalyst particle-supporting carbon material be supported on the carbon support material in a “highly dispersed state” and affixed state. Here, a “highly dispersed state” is the state where, to enable oxidizing gas to diffuse and water to be drained, the metal catalyst particles are dispersed on the carbon support material at a certain distance from each other and so as not to be separated more than necessary. Hereinafter, in this Description, the carbon material serving as the support at which the metal catalyst particles are supported will be called a “carbon support material”.
Therefore, to provide a solid polymer type fuel cell excellent in power generation performance under highly moistened conditions, the characteristics sought from a carbon support material are as follows:    (a) few hydrophilic functional groups at the carbon support material.    (b) carbon support material having large surface area for enabling carbon support material to support metal catalyst particles in a highly dispersed state.    (c) carbon support material having a suitable number of sites for affixing the metal catalyst particles at the carbon support material in a highly dispersed state.
Specifically, the characteristic of (a) requires reduction of the hydroxyl groups and carboxyl groups and other hydrophilic functional groups containing hydrogen at the ends of the functional groups at the carbon support material and suppression of flooding. Further, the characteristic of (b) requires giving the carbon support material a large surface area so as to enable oxidizing gas to diffuse and water to be drained and to cause the metal catalyst particles to be dispersed so as to be a certain distance from each other and so as not to be unnecessarily separated. Furthermore, the characteristic of (c) requires supporting the metal catalyst particles at the carbon support material in a highly dispersed state by increasing the surface area of the carbon support material and also suitably providing sites for affixing the metal catalyst particles at the carbon support material in a highly dispersed state. Even if the surface area of the carbon support material is large, if the number of sites for affixing the metal catalyst particles is small, the metal catalyst particles cannot be made to be supported at the carbon support material in a highly dispersed state. Therefore, the above characteristics of (a), (b), and (c) are matters necessary and essential which have to be achieved in order to provide a solid polymer type fuel cell excellent in power generation performance under highly moistened conditions.
Therefore, in the past as well, several attempts have been made to develop solid polymer type fuel cells excellent in power generation performance under highly moistened conditions. For example, PLT 1 proposes as a method for achieving the above characteristic of (a) introducing modifying groups comprised of hydrophobic functional groups of fluorinated groups at the surface of a carbon support material comprised of carbon black or other carbon particles and thereby reducing the occurrence of flooding.
Further, PLT 2 proposes a solid polymer type fuel cell using commercially available activated carbon or carbon black or another carbon support material to optimize the structure of the catalyst layer for good gas diffusion and thereby prevent water generated on the metal catalyst particles from closing the diffusion paths and realize high cell performance without regard as to the moistened conditions.
Furthermore, other than the approaches of the above PLTs 1 and 2, PLT 3 discloses a carbon support material produced by the following method as a material having the above characteristics of (b) and (c). That is, it discloses a method of production of a solid polymer type fuel cell-use carbon support material comprising a step of blowing acetylene gas into a solution containing a metal or metal salt and forming a metal acetylide (acetylide forming step), a first heat treatment step of heating the metal acetylide at a temperature of 60 to 80° C. for 12 hours or more to make the metal precipitate and be included as metal particles and prepare a metal particle-containing intermediate, a second heat treatment step of heating the metal particle-containing intermediate at a temperature of 160 to 200° C. for 10 to 30 minutes and ejecting the metal particles from the metal particle-containing intermediate to obtain a carbon material intermediate, a dissolution and washing treatment step bringing the carbon material intermediate obtained by the second heat treatment step into contact with a nitric acid aqueous solution, in particular concentrated nitric acid, and dissolving away the ejected metal particles and other unstable carbon compounds to clean the carbon material intermediate (washing treatment step), and a third heat treatment step of heating the carbon material intermediate cleaned in the dissolution and washing treatment step in a vacuum, in an inert gas atmosphere, or in an air atmosphere, at a temperature of 180 to 200° C. for 24 to 48 hours to obtain the carbon support material.